RAID has a backup unit (battery) for preparing for a case in which electric powers supply cannot be carried out from an outside of a system such as power failure. In a case where the power failure is generated, in order to protect data held by a volatile memory mounted to a controller module, the RAID carries out the electric power supply from the backup unit to the volatile memory. Until the RAID recovers from the power failure, the backup unit keeps supplying the electric power to the volatile memory. Thus, the volatile memory keeps holding the data.
However, a limit exists on a power supply capacity of the backup unit. For that reason, a data guarantee of the volatile memory depends on an electric capacity of the backup unit. If the capacity of the backup unit is increased, a reliability of the data guarantee can be accordingly increased. However, if the number of the backup units is increased in the RAID, new problems occur such as an increase in costs and an increase in size of a RAID apparatus. Also, after the recovery from the power failure, until the backup unit is recharged, a guarantee for a backup cannot be made, and therefore a RAID system is in a write through state (for reporting a completion when the write into a disk is done). As a result, a problem also occurs in which a processing ability of the RAID system is considerably decreased.
Therefore, for the controller module, a backup method of saving the date held by the volatile memory into a nonvolatile memory is conceived, and a problem is how the data is certainly saved into the nonvolatile memory.
The following documents exist with regard to a backup control in the RAID system.
[Patent Document] Japanese Laid-open Patent Publication No. 05-346889, Japanese Laid-open Patent Publication No. 2006-277395, Japanese Laid-open Patent Publication No. 2004-318465, and Japanese Laid-open Patent Publication No. 09-305491.